Зображення сторінки

for “washing," if practical, have it fully charged. Otherwise, if the plates are badly sulphated, they are likely to throw down considerable sediment on the charge after the cleaning is completed.

There have been many complaints of lack of capacity from batteries after washing. Almost without exception this is found to be due to lack of a completė charge following the cleaning. The plates are frequently in a sulphated condition when dismantled, and in any case are exposed to the air during the cleaning process, and thus lose more or less of their charge. When reassembled, they consequently need a very complete charge, and in some cases the equivalent of the initial charge, and unless this charge is given the cells will not show capacity and will soon give trouble again. This charge should be as complete as that described elsewhere in connection with the initial charge.

Dangers of Flushing.--"Flushing," or replacing evaporation in cells with electrolyte instead of water, is a most common mistake. The plates of a storage battery must always be kept covered with electrolyte, but the evaporation must be replaced with pure water only. There seems to be a more or less general tendency to confuse the electrolyte of a storage battery with that of a primary cell. The latter becomes weakened as the cell discharges and eventually requires renewal. With the storage battery, however, this is not the case, at least to anything like the same degree, and unless acid is actually lost through slopping or a broken jar it should not be necessary to add anything but water to the cells between cleanings. Acid goes into the plates during discharge, but with proper charging it will all be driven out again, so that there will be practically no loss in the specific gravity readings, or at least one so slight that it does not require adjustment between cleanings. Thus, unless some of the electrolyte has actually been lost, if the specific gravity readings are low, it is an indication that something is wrong; but the trouble is not that the readings are low, but that something is causing them to be low, and the proper thing to do is to remove the cause and not try to cover it up by doctoring the indicator. The acid is in the cells, and if it does not show in the readings it must be in the form

of sulphate, and the proper thing to do is to remove the cause of the sulphation if there is one, and then, with proper charging, drive the acid out of the plates and the specific gravity readings will then come back to the proper point. The too-frequent practice in such cases is to add electrolyte to the cells in order to bring up the readings which, as already explained, are only the indication of the trouble, and this further aggravates the condition, until finally the plates become so sulphated that lack of capacity causes a complaint. This practice of adding electrolyte to cells instead of water seems to be becoming more and more common.

Sulphation, Cause and Cure.—When plates are sulphated, to restore them to their original condition it is necessary that the battery be given a long, slow charge at about a quarter or a third of the normal charging rate. This should be continued until the electrolyte has reached the proper specific gravity and the voltage has attained its maximum.

It should be understood that sulphating is a normal as well as an abnormal process in the charge and discharge of storage batteries, and the difference is in the degree, not the process. The abnormal condition is that ordinarily referred to by the term. In normal service sulphating does not reach the point where it is difficult to reduce, but if carried too far, the condition becomes so complete that it is difficult to reduce and injury results. A very crude method of illustrating the different degrees of sulphating is to consider it as beginning in individual particles uniformly distributed throughout the active material. Each particle of sulphate is then entirely surrounded by active material. The sulphate itself is a non-conductor, but, being surrounded by active material, the current can reach it from all sides and it is easily reduced. This is normal sulphate. As the action goes further the particles of sulphate become larger and join together and their outside conducting surface is greatly reduced in comparison with their volume, so that it becomes increasingly difficult to reduce them, and we have abnormal sulphate.

Slow Charge Cures Sulphation.—The general cure for sulphating is charging, so that a cell, having been mechanically restored, the electrical restoration consists simply in the proper charging.

Sulphate reduces slowly, and on this account it is a good plan to use a rather low current rate. High rates cause excessive gassing, heating, and do not hasten the process appreciably, so that it is the safer as well as the more efficient plan to go slowly. A good rate is about one-fifth normal. The length of charge will depend upon the degree of sulphating. In one actual case it required three months' charging night and day to complete the operation, but this was, of course, an exceptional one. The aim should be to continue until careful voltage and gravity readings show no further increase for at least ten hours and an absolute maximum has been reached. In serious cases it may be advisable to even exceed this time in order to make absolutely sure that all sulphate is reduced, and where there is any question it is much safer to charge too long rather than to risk cutting off too soon. A partial charge is only a temporary expedient; the cell, still being sulphated, will drop behind again.

The Gould Storage Battery Company give a number of characteristics by which cells in poor condition may be recognized in addition to the usual hydrometer readings. The common causes of plate deterioration are also given. The plates may be of poor color; the color of a wet positive plate in good condition varies from a rich dark brown (almost black) if the plate is fully charged to a reddish, fairly dark brown if discharged. A light grayish coating on the positive plates is not a bad indication, if by rubbing with a clean stick or piece of hard rubber a good color is evident immediately under the surface. The color is much lighter for dried plates. The wet negatives are of a light slate gray if charged and somewhat darker if discharged. When dry they are considerably lighter, and may even be somewhat yellowish if allowed to heat in drying. If the color of the plates is not as described they are probably considerably sulphated. If the cell voltage is markedly lower on discharge or higher on charge than it should be, sulphating is also indicated. If the acid strength is low, the cell should be investigated for short circuits or sulphated plates. Always be sure that the sediment does not touch the plates. It must be removed as soon as there is danger of this occurring

Causes of Plate Deterioration.—Plates may get in poor condition from the following causes: 1st, Impure Electrolyte.Either a poor quality used at the start, or through the use of impure water or through foreign substances getting into the cells. The remedy in this case, if the plates are physically in fair condition, is to replace the old electrolyte with new, the cells being in a discharged condition, and then thoroughly charge the battery. 2nd, Short Circuits.—These are not frequent if the sediment is removed before it touches the plates, as the wash of the electrolyte in most vehicle batteries resulting from the movement of the car would tend to free them. If they do occur, the cell should be completely dismantled, the plates straightened and the cell assembled again, the separators being completely replaced. The cell should then be thoroughly charged. 3rd, High Temperature.At temperatures above 100° F. corrosion is quite rapid, and this limit must not be exceeded. If possible, the temperature should not exceed 90° F. The positives may be sulphated considerably from this cause and the plates grown abnormally and distorted. If they are thoroughly corroded they must be replaced; if not, they should be straightened and thoroughly charged. The conditions should be changed so that the battery will not again be subjected to the high temperature. 4th, Standing Discharged.The positive plates especially may be badly sulphated from this

The indication of this condition is a light color of the positive plates, possibly with blotches of a grayish color. Tlie remedy in this case is also complete charge, though care must be taken that too much active material is not thrown off during the charge. Under these conditions the active material is granular and non-cohesive, so care is needed in charging and discharging to restore the plates to efficiency.

Value of Cadmium Readings.—It is possible to make tests to determine the relative capacity of the positive and negative plates of any cell by means of a neutral electrode. Cadmium is well adapted for this purpose, and while such tests are not ordinarily made with small batteries they are very useful in determining the condition of large cells. The cadmium element consists of a stick of that material about the size of a lead pencil and 6 inches


long, which is inserted into a soft rubber tube. Before inserting the cadmium stick, the rubber tube should be perforated with a number of holes about 1/16 of an inch in diameter. The rubber tube should extend at least 18 of an inch beyond one end of the cadmium element. A small flexible rubber-covered copper wire should be soldered to one end of the cadmium stick. This wire may be joined to the negative pole of a voltmeter. A regu


Fig. 21.-Apparatus Used in Making Cadmium Tests.

lar contact "sticker," or "stabber," such as shown at Fig. 21, is connected to the positive pole of the voltmeter.

After the total cell voltage throughout the battery is taken in the regular way, cadmium readings can be taken by inserting the rubber-covered end of the cadmium into the acid of the first cell, being sure that the bare cadmium stick does not touch either of the plate groups. Press the "sticker” leading to the positive pole of the voltmeter against the positive cross bar of the cell and note the reading. The difference between the reading of the positive group and the cadmium and the total cell voltage already taken will represent approximately the negative cadmium reading. For example, if the positive cadmium reading is two volts and the regular cell voltage 1.85, the negative cadmium would be 0.15. A positive is discharged when its cadmium reading at its

« НазадПродовжити »